The present invention relates to electron microscopes and more particularly to an electron microscope suitable for automatic adjustment of the optical axes of a radiation lens system and an image-forming lens system.
Generally, when the optical axis of the radiation lens system is tilted with respect to the optical axis of the image-forming lens system, optical axis adjustment called current axis alignment or voltage axis alignment is conducted.
The current axis alignment is effected by utilizing the fact that when objective lens current is increased or decreased, an enlarged image is moved in the circumferential direction about a certain position (current center) as shown in FIG. 3A, and it is an alignment method wherein the operator finds the current center when the enlarged image is moved in the circumferential direction by increasing or decreasing the objective lens current and the deflector is adjusted such that the current center coincides with the center of view field.
On the other hand, the voltage axis alignment is effected by utilizing the fact that when accelerating voltage of the electron gun is increased or decreased, an enlarged image is moved in the radial direction from a certain position (voltage center) as shown in FIG. 3B) and it is an alignment method wherein the operator finds the voltage center when the enlarged image is moved in the radial direction by increasing or decreasing the accelerating voltage and the deflector is adjusted such that the voltage center coincides with the center of view field.
Incidentally, with axis alignment as such, the enlarged image is blurred as it moves and therefore, accurate finding of the current (voltage) center is difficult to require the operator to have high skill.
To solve such problems, in a prior art, for example, as described in the specification of U.S. Pat. No. 4,788,425, a technique has been proposed wherein as objective lens current or accelerating voltage is increased or decreased, final images are stored in an image memory and integrated or averaged to provide an image which is displayed.
Since through this the operator is allowed to easily find the current center or voltage center in accordance with the degree of blurr of image, axis alignment can be carried out easily and accurately.
Further, in the aforementioned prior art, a technique is also proposed wherein when an image is displayed, a marker is also displayed concurrently and optical axis alignment is effected by making the marker coincident with the current center or voltage center of a final image.
Also, in JP-A-62-143351, a technique is proposed wherein an amount of electric charge irradiated on a specimen is detected and the optical axis is automatically adjusted by moving the lens system in a direction vertical to the optical axis such that the electric charge amount exhibits a maximum value.
In the prior art described in the U.S. Pat. No. 4,788,425 specification, when finding the current center or voltage center, judgment of the operator is needed and therefore automation could not be planned.
Also, the prior art described in JP-A-62-143351 faced a problem that because of the need of a detector for detection of the electric charge amount, the construction is complicated.
In addition, in order to detect the position at which the electric charge amount exhibits a maximum value, the lens system has to be sufficiently scanned in the direction vertical to the optical axis, raising a problem that much time is needed.